This invention relates to an apparatus for improving the viewing quality of a television receiver picture. Specifically this invention relates to altering the high frequency content in a television receiver luminance signal in accordance to its content, to provide consistently higher quality pictures.
It is known in the art that picture resolution in a television receiver may be improved by frequency compensating (known also as "peaking") the luminance signal path. The luminance signal suffers from a loss of high frequency content as a result of limited transmission bandwidth and limited receiver bandwidth which reduce picture resolution. Prior art techniques for increasing the effective bandwidth of received video signals include the use of a peaking amplifier in the luminance signal path. The peaking amplifier provides a means for varying the slope of the gain versus frequency characteristics of the luminance signal path. By varying the gain-frequency characteristics of the luminance signal path, the higher frequencies may be amplified or "peaked" to a greater extent than the lower frequencies. This increases the amplitude of the high frequency components in the luminance signal, resulting in improved picture resolution.
Excessive frequency compensation may also deteriorate picture resolution. When different channels are selected or different cameras are used at the television studio, the signal peaking requirements change. Consequently, the operator must frequently adjust the peaking control to establish the right amount of high frequency video content in the luminance signal to avoid either excessive or inadequate signal peaking
Optimum peaking for a given scene depends upon the amount of detail in the picture, and the amount of high frequency noise in the luminance signal. Also, it is desirable to peak the higher level white areas of a picture to a lesser extend than the gray areas of the picture.
When viewing a good quality television picture having adequate detail and low noise content, it may be desirable to adjust the receiver luminance signal frequency response to "flat", or to adjust its response to provide a small amount of peaking of the higher frequencies containing the detail information to add "crispening" for a more pleasing picture.
When a "soft" picture signal is received which has little detail, it has been observed that a higher level of peaking at a lower frequency will crispen the picture without substantially increasing the visibility of high frequency noise.
Pictures which have objectionable levels of upper video spectrum noise are usually improved by decreasing the amount of peaking in the luminance channel which reduces the visibility of noise in the picture.
Conventional peaking systems apply equal amounts of peaking to the white and gray areas of the picture. While it may be desirable to provide a certain level of peaking to the gray area of a picture to improve its detail, the higher level whites, such as white alpha numerics or white small area details are also similarly peaked. This causes an excessive picture tube current resulting in defocussing and loss of picture detail in these high-light areas. It is desirable that a dynamic form of peaking be used to provide the gray level crispening without the high light defocussing.